Background. Availability of different tyrosine
kinase inhibitors (TKIs) with distinct anti-leukemic potency
enables the optimization of current therapeutic regimens; however,
some patients lose their therapy responses and acquire TKIs
resistance. Aims. In this study, a single center
experience with monitoring of BCR-ABL1 kinase domain (KD)
mutations is described and, particularly, the impact of the
treatment administration on mutations selection is discussed.
Methods. The CML patients treated with TKIs at our
institution during the years 2003 - 2011 were included in this
study. Peripheral blood and/or bone marrow samples were collected,
processed and analyzed using the published procedures; detection of
BCR-ABL1 KD mutations was performed by direct sequencing.
Results. First, we evaluated the impact of non-TKI
pretreatment on: a) spectrum of BCR-ABL1 KD mutations; b)
their frequencies and c) time to mutation detection after the
administration of a first-line TKI. Our data shows that
pretreatment with non-specific non-TKI drugs does not
preferentially select the BCR-ABL1 KD mutations, as there
was no difference between the frequency of T315I or p-loop
mutations compared to mutations in other KD regions (52. 7% vs. 47.
3%). In contrast, the imatinib (IMA) as first-line therapy led to
clear predominance of T315I or p-loop mutations (90. 9% vs. 18. 2%
of mutations located in other KD regions). In addition, median time
to the detected p-loop mutations was substantially shorter in
patients treated with IMA first-line compared to patients who were
pretreated (16 months (3 - 41) vs. 46 months (20 - 109), p = 0.
005). Second, we analyzed the impact of administered TKIs on
appearance and selection of mutations in patients that were
recurrently resistant to TKIs therapy. Among the patients with
initial BCR-ABL1 KD mutation, 32. 0% developed a second
mutations after the change of TKIs treatment. The more potent
second-line TKIs therapy caused the elimination of most of the
initial IMA resistant mutations; however, several mutations
resistant to secondline therapy subsequently emerged, with
considerably shorter time compared to the initially detected ones
(9 months (1 - 25) vs. 21 months (3 - 109), p = 0. 007). Moreover,
after the therapeutic intervention with third-line TKIs, similar
consequences were observed. Third, we confirm the previously
described poor prognosis of CML patients with mutated
BCR-ABL1 KD, since 40. 0% of our CML patients that
harbored BCR-ABL1 KD mutation died during the treatment
with TKIs. Moreover, 27. 8% of patients who are still on treatment
have already progressed. Last, our data confirms the unique
position of T315I mutation with respect to its strong resistance to
the currently approved TKIs. Conclusions. Based on
a “real-life”data described in this study, for some of the CML
patients that harbor BCR-ABL1 KD mutations, it may seem
that therapy itself consecutively leads to its failure, selecting
thus the most resistant mutations under the selective pressure of
applied therapy regimen (Figure 1). Note that formation and
selection of mutations, that could reflect the genomic instability,
is not a single process, thus all aspects must be taken into
account.

Acknowledgements. Supported by The
CzEch Leukemia Study Group for
Life (CELL) and grants MSM0021622430 and
MUNI/A/0784/2011.